It is narrated from Sayyiduna Sulayman bin Surad (may Allah be pleased with him) that he said: "Two angels came to Muhammad (peace and blessings be upon him). One of them said: 'Recite the Qur'an in one mode (harf).' The other said: 'Give him more.' He kept asking for more until he said: 'Recite the Qur'an in seven modes (ahruf).'"
Hadith Referenceالمعجم الأوسط / باب الألف / 1167
Hadith Takhrij«أخرجه النسائي فى «السنن الكبرى» برقم: 10438، والطحاوي فى «شرح مشكل الآثار» برقم: 3114، 3115»